1. Field of the Invention
This invention relates to a brushless DC motor in which current paths to polyphase coils are electronically switched by transistors.
2. Description of the Prior Art
The brushless DC motor, which does not use the brush-commutator contact has many advantages such as no occurrance of commutation noise, long life and high reliability, and has been widely used for the audio and visual equipments. In the brushless DC motor, the current paths are switched by a non-contact position detector and the drive transistors which is activated selectively in correspondence with the output signals of the position detector. The output currents of the drive transistors are controlled in such a way that the supply current to the polyphase coils becomes a predetermined value (the value corresponding to a command signal). As the result, the temperature of the drive transistors rises due to the power dissipation (collection loss) of the drive transistors, which causes the thermal breakdown or the decline of life of the drive transistors. In a case that the drive transistors and resistors are integrated on a single silicon chip, the power dissipation of the drive transistors causes the great increase of heat in the silicon chip, so that the integrated circuit (IC) is destroyed by the thermal breakdown. Especially, when the temperature of the environment where the motor is used is higher, the inner temperature becomes higher with respect to the same power dissipation. So the limitation against the environmental temperature for using the brushless DC motor is severe, which restricts the practical use of the brushless DC motor.